Ti-A1 intermetallic compound sheet and method of producing same

ABSTRACT

The present invention provides a Ti--Al intermetallic compound sheet of a thickness in the range of 0.25 to 2.5 mm formed of a Ti--Al intermetallic compound of 40 to 53 atomic percent of Ti, 0.1 to 3 atomic percent of at least one of material selected from the group consisting of Cr, Mn, V and Fe, and the balance of Al, and a Ti--Al intermetallic compound sheet producing method comprising the steps of pouring a molten Ti--Al intermetallic compound of the foregoing composition into the mold of a twin drum continuous casting machine, casting and rapidly solidifying the molten Ti--Al intermetallic compound to produce a thin cast plate of a thickness in the range of 0.25 to 2.5 mm and, when necessary, subjecting the thin cast plate to annealing and HIP treating. The Ti--Al intermetallic compound sheet has excellent mechanical and surface properties.

TECHNICAL FIELD

The present invention relates to a Ti--Al intermetallic compound sheetand a method of producing the same, and more particularly, provides theTi--Al intermetallic compound sheet of a structural material havinglight weight, heatresistance, high temperature strength, and othersuperior properties suitable for aeronautical and astronauticalpurposes, and a method of producing such the Ti--Al intermetalliccompound sheet.

BACKGROUND ART

It is well known that the Ti--Al intermetallic compound has fairly machthe maximum high temperature specific strength of metallic materials,and further, is high in corrosion resistance and light in weight.Metallurgical Transaction, Vol. 6A, p.1991 (1975) reported that ahightemperature strength of 40 kg/mm² was obtained at 800° C. Therefore,it has been considered optimal to use these characteristics and applythe Ti--Al intermetallic compound to gas turbine components, valves andpistons of automobile engines and apply them to dies used at hightemperature, bearing parts, etc.

The Ti--Al intermetallic compound has a composition range in a phasediagram, and at a Ti content of 40 to 52 atomic percent and an Alcontent of 60 to 48 atomic percent in a heat equilibrium state becomes asingle phase of an L1₀ structure (basically, a face-centered tetragonalstructure, but layers of Ti and Al are arranged intersectingly in the<001> direction). It has been found that an abnormal strengtheningphenomenon occurs whereby the strength of the Ti--Al intermetalliccompound in a single crystal state increase with an elevation of thetemperature. It is known that the strength of the Ti--Al intermetalliccompound in a polycrystalline state is not lowered under a hightemperature, but the polycrystalline Ti--Al intermetallic compound hasdisadvantages of a low ductility in the temperature range of roomtemperature to about 700° C. (Japanese Examined Patent Publication No.Sho 59-581), and the hot rolling of the polycrstalline Ti--Alintermetallic compound is very difficult. Accordingly, near-net-shapecasting techniques which gives close to the final product must beemployed to produce Ti--Al intermetallic compound sheets.

Recently, rapid progress has been made in near-net-shape castingtechniques and, particularly when processing metallic materials, havebeen progressively applied to producing stainless steel sheet etc.Various casting methods as sheet manufacturing techniques have beenproposed, and among those previously proposed casting methods, it hasbeen found that a twin-drum method is suitable for producing acontinuous sheet having a uniform thickness.

As an exemplary application of the foregoing techniques to anintermetallic compounds, there is known the example of a Ni--Alintermetallic compound (Ni₃ Al) having an improved ductility by adding asmall amount of boron. This example is reported in the internationalconference being held in November 1988, on "Casting of Near-Net-ShapeProducts" (the proceedings of an International Symposium on Casting ofNear Net Shape Products, pp.315-333, issued by The MetallurgicalSociety.). A Ti--Al intermetallic compound sheet producing method isalso disclose in Japanese Patent Application No. Hei 1-50649.

Although the application of a direct sheet process to the obtaining ofnear-net-shape products has the advantage of a curtailment of themanufacturing processes, a rapid cooling of the cast sheet in the directsheet manufacturing process produces defects, such as surface cracks andporosities, in the sheet.

Accordingly, it is important to eliminate those defects in sheetsproduced by direct casting, to ensure sound and highly reliable sheetproducts.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method of producing asheet having desired characteristics by the near-net-shape casting of aTi--Al intermetallic compound having an optimum composition and anoptimum crystal structure. Although the direct casting method ofproducing a sheet in a near net shape has significant advantagesincluding a curtailment of the processes, the direct casting method hasdisadvantages in that the sheet produced by the same method has aninferior workability and mechanical properties because the method doesnot include a forging process, which is effective for a satisfactoryadjustment and control of the crystal structure of material forming thesheet.

Accordingly, it is important to achieve the optimum adjustment andcontrol of the crystal structure in the casting process for forming anoptimum crystal structure, to provide by direct casting a sheet producthaving satisfactory characteristics, such as an excellent workabilityand mechanical properties, and thus ensure a highly reliable sheetproduct.

Another object of the present invention is to provide a techniquecapable of preventing defects including surface cracks and porositieswhen producing a near-net-shape product by direct casting.

The inventors of the present invention made a study of ways in which toachieve the foregoing objects, and created the present invention on thebasis of findings obtained by the study that a Ti--Al intermetalliccompound having a specific composition and a specific crystal structuremust be used to solve the problems in the direct near-net-shape castingmethod, and that the application of specific casting conditions, a heattreatment process subsequent to a casting process, and specific processsubsequent to the heat treatment process, to the direct neat-net-shapecasting method is effective.

A gist of the present invention is a cast sheet of a thickness in therange of 0.25 to 2.5 mm formed of a Ti--Al intermetallic compound of aternary system containing Ti in a content in the range of 40 to 53atomic percent, at least one of material selected from the groupconsisting of Cr, Mn, V and Fe in a content in the range of 0.1 to 3atomic percent, and the balance of Al and unavoidable impurities, andformed by processing a cast plate having, in an as-cast state, acolumnar crystal structure growing from the opposite surfaces toward thecentral portions or a mixed structure of the columnar crystal structureand an equiaxed crystal structure existing in a vicinity of a centralportion of the cast plate.

Another gist of the present invention is a method of producing a sheethaving an excellent quality without surface defects including surfacecracks and porosities, comprising the steps of forming a thin cast plateby casting the Ti--Al intermetallic compound of the foregoingcomposition in a mold by a twin-drum continuous casting machine, coolingthe thin cast plate to a room temperature by furnace cooling, ifnecessary, after holding the thin cast plate at a temperature in therange of 800° to 1000° C. for a predetermined time, and pressing thethin cast plate by a hot isostatic pressing process.

A cast structure favorable to plastic working will be describedhereinafter.

In accordance with the present invention, the ascast solidified castplate has the columnar crystal structure growing from the oppositesurfaces toward the central portion of the mixed structure of thecolumnar crystal structure and the equiaxed crystal structure existingin the vicinity of the central portion of the cast plate. The columnarcrystal structure has the following conformation.

In the Ti--Al intermetallic compound, a dual-phase eutectic texture of aγ-phase (Ti--Al intermetallic compound and L1₀ structure) and an α₂-phase (Ti₃ Al intermetallic compound and D0₁₉ structure) can beobtained by changing the ratio of composition of Ti and Al. When theTi--Al intermetallic compound of the foregoing composition consists of40 to 53 atomic percent of Ti, 0.1 to 3 atomic percent of a tertiaryelement, and the balance of Al, a hexagonal crystal compound firstcrystallizes during the solidification from the molten state, and thehexagonal crystal crystallizes selectively with the {0001} face isarranged in parallel to the sheet face, namely, with the <0001>direction is arranged in parallel to the sheet thickness direction, whenthe molten compound is solidified at a suitable cooling rate. However,in a compound of this range of composition, the hexagonal crystalsstable just under the solidification point, and a regular structuralchange into the γ-phase (L1₀ structure) occurs. At the time of thesestructural change, the <111> crystal orientation of the L1₀ structurebecomes parallel to the <0001> direction of the hexagonal crystals.Accordingly, a Ti--Al intermetallic compound sheet of a compositionhaving Ti and Al contents approximately equal to the Ti--Alstoichiometric ratio having the required texture, i.e., a texture withthe <111> crystal orientation preferentially coinciding with thedirection of thickness of the cast plate can be produced by cooling acast Ti--Al intermetallic compound at an appropriate cooling rate. If0.1 to 3.0 atomic percent of one or a plurality of the tertiary element,such as Cr, Mn, V or Fe, is added to this system, the crystal structureis made to shrink and become isotropic and the casting structure is madefiner and a required strength over the temperature range of a roomtemperature to 1000° C. is secured without detriment to the requiredtexture.

The foregoing effect is not obtained if the content of the tertiaryelement is less than 0.1 atomic percent, and the tertiary elements fromcompounds which deteriorate the ductility of the cast plate if thecontent of the additive element is greater than 3.0 atomic percent.Therefore, the content of the tertiary element or elements must be inthe range of 0.1 to 3.0 atomic percent.

The hexagonal crystals of the cast plate are not formed in thepreferential crystal orientation and the regular structural change forthe L1₀ structure does not occur even if the cast plate is cooled at ahighest cooling rate if the thickness of the cast plate is less than0.25 mm, and a random nucleation of crystals occurs in the centralportion of the cast plate and the desired structure is not formed evenif the cast plate is cooled at a highest cooling rate if the thicknessof the cast plate is grater than 2.5 mm. Therefore, the thickness of thecast plate must be in the range of 0.25 to 2.5 mm.

A method of casting such a thin cast plate will be describedhereinafter.

A twin-drum continuous casting machine (hereinafter referred to simplyas "casting machine"), in general has two cooling drums disposed withtheir axis in parallel to each other for rotation in oppositedirections, respectively, and side dams disposed contiguously with theopposite ends of the cooling drums, respectively, to form a basin (mold)in combination with the cooling drums. A molten metal poured into thebasin is cast to form a thin cast plate while the molten metal is cooledby the rotating cooling drums.

According to the present invention, a molten Ti--Al intermetalliccompound is poured into the basin and the same is cast to produce a thincast plate. Since the Ti-- Al intermetallic compound has a lowductility, cracks are liable to form in the thin cast plate duringsolidification and cooling, the formation of oxides, which causeirregular solidification, in the meniscus must be suppressed. Therefore,the Ti--Al intermetallic compound must be melted and cast in theatmosphere of an inert gas, such as an Ar gas or He gas.

The directly cast thin cast plate is cooled slowly by, for example,furnace cooling, immediately after leaving the mold. The thin cast platemay be held at a predetermined temperature for a predetermined time ormay be subjected to HIP, if necessary.

Thus, a sheet of an excellent quality having neither surface cracks norporosities can be produced.

When casting the thin cast plate by such a process, it is desirable tocool the thin cast plate at a cooling rate in the range of 10² ° C./secto 10⁵ ° C./sec. The cooling rate of 10⁵ ° C./sec is the upper limit ofcooling rate for solidifying the Ti--Al intermetallic compound inhexagonal crystals and for causing a regular structural change to forman L1₀ structure. If the cooling rate is less than 10² ° C./sec, arandom nucleation of crystals occurs and the preferred nature of thecrystal orientation is lost.

The thin cast plate is cooled at a cooling rate of up to 200° C./hr to atemperature not higher than 200° C., to prevent the development ofsurface cracks. Nevertheless, the thin cast plate may be held at atemperature in the range of 800° to 1000° C. for a time of in the rangeof 1 to 20 minutes after solidification, to curtail the time requiredfor slow cooling. The above holding temperature is a necessarytemperature to prevent the development of cracks due to thermal stress.The holding means are as follows, namely, a heating furnace may providednear a position where the thin cast plate leaves the mold or the coolingdrums may be stopped to solidify the molten metal partly in a bulk format above the cooling drums before the thin cast plate leaves the moldcompletely, so that thin cast plate is suspended from above the coolingdrums.

The HIP treatment is carried out to crush to porosities (voids) in thecast plate, in which the cast plate is held at a temperature in therange of 1000° to 1400° C. (a temperature below the melting point) for atime in the range of ten minutes to one hour in an atmosphere of apressure not lower than 1000 atm.

Thus, a Ti--Al intermetallic compound sheet having excellent mechanicalproperties and not having surface and internal defects can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional side view of an apparatus for carryingout the present invention;

FIG. 2 is a metallographic photograph of a section of a cast plateproduced by a method in accordance with the present invention takenalong a casting direction;

FIGS. 3(A) and 3(B) are photographs of the surface of a cast plate inaccordance with the present invention cooled by furnace cooling aftercasting, and the surface of a cast plate in accordance with the presentinvention cooled by natural cooling after casting, respectively; and

FIGS. 4(A) and 4(B) are photographs of a section of a Ti--Alintermetallic compound cast plate after being treated by a HIP, and asection of the same Ti--Al intermetallic compound cast plate beforesubjection the same to the HIP, respectively.

BEST MODE OF CARRYING OUT THE INVENTION

A best mode of carrying out the present invention will be described withreference to preferred embodiments thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Each of mixtures of aluminum, titanium sponge and other element orelements, such as Cr, Mn, V or Fe, respectively having compositionsshown in Table 1 was melted in a plasma arc furnace to obtain motheralloys.

The molten mother alloys were cast by a casting machine shown in FIG. 1to produce thin cast plates. As shown in FIG. 1, the casting machinecomprises a turndish 2 for uniformly pouring a molten metal, disposedunder a crucible 1 for melting a Ti--Al intermetallic compound, a basin5 comprised by side dams 4 and the cooling drums 3 (mold) and disposedunder the turndish 2, an atmosphere adjusting vessel 7 containing theforegoing components, an inert gas supply mechanism 8, and a dischargemechanism 9.

                                      TABLE 1    __________________________________________________________________________                          Weight                    Drum  of                    supporting                          mother                              Secondary  Thick-    Sample         Composition                    force alloy                              cooling                                    Surface                                         ness                                             Continuous                                                    Re-    Nos. (at. %)    (kgf) (g) (°C., min)                                    property                                         (mm)                                             length (mm)                                                    marks    __________________________________________________________________________    1    52Ti48Al   1000  3500                              1000, 30                                    Cross                                         1.51                                             150-680-850-                                                    Com-                                    mark     440    para-    2    52Ti48Al   1000  2000                              1000, 10                                    Cross                                         1.58                                             2070   tive                                    mark            exam-    3    52Ti48Al   1000  2100                              950, 10                                    Good 1.54                                             2170-380-                                                    ples                                             230-30    4    52Ti48Al   1000  2000                              900, 10                                    Good 1.58                                             2310-230    5    52Ti48Al   1000  2000                              850, 10                                    Good 1.55                                             2500    6    52Ti48Al   1000  2000                              800, 10                                    Good 1.55                                             1680-480    7    50Ti48Al2Cr                    1000  3500                              1000, 10                                    Good 1.51                                             2400   Exam-    8    50Ti48AlMn 1000  2000                              1000, 10                                    Good 1.58                                             2070   ples    9    50Ti48Al2V 1000  2100                              1000, 10                                    Good 1.54                                             2220-380    10   50Ti48AlFe 1000  2000                              1000, 10                                    Good 1.58                                             2300-200    11   50Ti47Al3Cr                    1000  2000                              1000, 10                                    Good 1.55                                             2400    12   50Ti47Al3Mn                    1000  2000                              1000, 10                                    Good 1.55                                             2480    13   50Ti47Al1.5Cr1.5Mn                    1000  2000                              1000, 10                                    Good 1.55                                             2000    __________________________________________________________________________

Each of the mother alloys of a weight in the range of 2000 to 3500 gshown in Table 1 was poured into the crucible 1 and was melted in an Aratmosphere by heating the mother alloy at 1600° C., the temperature ofthe molten mother alloy was adjusted to 1500° C., and then the moltenmother metal was poured through the opening of 4 mm in width and 95 mmin length formed in the turndish 2 into the basin 5. The cooling drums 3are a pair of drums of 300 mm in diameter and 100 mm in length formed ofa copper alloy. The cooling drums 3 are cooled internally, therefore themolten mother alloys were cooled rapidly for solidification under apredetermined force supporting by the drams and at a cooling rate of 10³° C./sec to produce continuous thin cast plates 6 respectively havingthickness tabulated in Table 1.

FIG. 2 is a photograph of a section structure of one of the thin castplates, i.e., Specimen No. 7, taken along the casting direction. Theas-cast solidification structure of said plates was consisted of onlycolumnar crystals oriented from the opposite surfaces of the thin castplate toward the central portion of the same or a mixed structureconsisting of the columnar crystals and equiaxed crystals formed in thecentral portion of the thin cast plate.

As stated above, the microstructure of the thin cast plate produced bythe method in accordance with the present invention was a refinedlaminated composite structure of structures with the preferentialorientation of the <111> crystal orientation of the L1₀ structure in thedirection of the thickness of the thin cast plate and of the <0001>direction of the D0₁₉ structures. Moreover, the tertiary element, suchas Cr, contained in the Ti--Al intermetallic compound, then the abovelaminated composite structure was very fine; the width of a layer ofeach L1₀ structure was 1000 Å and that of the D0₁₉ was 100 Å.

On the other hand, Specimen No. 1, which contains no tertiary element,also had a laminated microstructure, however, the width of a layer ofeach the component structures was 10000 Å and 1000 Å, and the laminatedstructure was coarse compared with the laminated structure of the thincast plate formed of the Ti--Al intermetallic compound in accordancewith the present invention.

The cast plate 6 delivered from the cooling drums 3, 3 was cooled at alow cooling rate of 1° C./sec in the atmosphere adjusting vessel 7, wasinserted in a furnace, not shown, and treated by a secondary coolingconditions shown in Table 1 at the furnace, and the furnace thendisconnected from the power source and the cast plate 6 was cooled to atemperature below 200° C. by furnace cooling.

Table 2 shows the mechanical properties (elongation (%)) at a roomtemperature and at a high temperature of the cast plates thus produced.The cast plates formed of Ti--Al intermetallic compounds in accordancewith the present invention have high elongations both at the roomtemperature and at the high temperature, compared with those ofcomparative examples.

FIGS. 3(A) and 3(B) show the surface properties of the cast plate inSpecimen No. 7 cooled respectively by furnace cooling and by naturalcooling after leaving the cooling drums. Few surface cracks were foundin the surface of the cast plate cooled at a relaxation cooling rate,whereas minute surface cracks were found in the surface of the castplate cooled by natural cooling.

The surface properties of the cast plates by furnace cooling of eachspecimen were shown in Table 1. The specimens in accordance with thepresent invention had satisfactory surface properties.

The cast plates were subjected to a HIP of 1000° C. and 1500 atm. aftercooling the same to a temperature below 200° C., and their rupturestress (three-point bending strength) was measured. Measured results areshown in Table 3. The rupture stress of specimens in accordance with thepresent invention were higher than that of the comparative examples, andit was confirmed that the HIP greatly enhances the rupture stress.

                  TABLE 2    ______________________________________                        Cold      Hot    Sam-                elongation                                  elong-    ple  Composition    (room tem-                                  ation    Nos. (at. %)        perature) (800° C.)                                         Remarks    ______________________________________    1    52Ti48Al       1.4       12     Compar-    2    52Ti48Al       1.5       13     ative    3    52Ti48Al       1.4       12     exam-    4    52Ti48Al       1.5       12     ples    5    52Ti48Al       1.4       12    6    52Ti48Al       1.5       11    7    50Ti48Al2Cr    1.9       20     Exam-    8    50Ti48Al2Mn    1.7       20     ples    9    50Ti48Al2V     1.7       18    10   50Ti48Al2Fe    1.7       17    11   50Ti47Al3Cr    1.9       22    12   50Ti47Al3Mn    1.8       21    13   50Ti47Al1.5Cr1.5Mn                        1.8       20    ______________________________________

                  TABLE 3    ______________________________________                  Rupture stress                  (kg/mm.sup.2)    Sample              As-cast    Nos.  Composition (at. %)                        (annealed)                                  After HIP                                          Remarks    ______________________________________    1     52Ti48Al      65.2      70.0    Compar-    6     52Ti48Al      55.3      69.0    ative                                          examples    7     50Ti48Al2Cr   79.5      97.3    Examples    8     50Ti48Al2Mn   77.9      85.8    9     50Ti48Al2V    75.2      90.7    10    50Ti48Al2Fe   76.8      100.2    11    50Ti47Al3Cr   65.0      102.4    12    50Ti47Al3Mn   76.9      89.2    13    50Ti47Al1.5Crl.5Mn                        75.0      100.0    ______________________________________

                  TABLE 4    ______________________________________                  Elongation                  (1200° C., 5 × 10.sup.-4 /sec)    Sample              AS-cast    Nos.  Composition (at. %)                        (annealed)                                  After HIP                                          Remarks    ______________________________________    1     52Ti48Al      20.0       30.0   Compar-                                          ative                                          example    7     50Ti48Al2Cr   30.0      100.5   Examples    11    50Ti47Al3Cr   25.0      102.0    ______________________________________

A specimen consisted of 50 atomic percent Ti and 50 atomic percent Alwas processed by a HIP of 1250° C. and 1500 atm. for one hour to examinethe porosities removing effect of the HIP. The result of this was shownin FIG. 4(A). It is known that almost all the porosities of the samebefore the HIP were removed by the HIP.

The hot workability (1200° C., strain rate of 5×10⁻⁴ /sec) of SpecimensNos. 7 and 11 containing Cr was examined. The elongation of thespecimens processed by the HIP was not less than 100%, which obviouslyis different from that of Specimen 1, i.e., a comparative example.

Thus, the present invention greatly improves the mechanical propertiesof the cast plates or processed sheets, which is inferred to be duemainly to the fining effect of the tertiary element on the texture ofthe Ti--Al intermetallic compound, the holding treatment of the castplate and the HIP treatment.

CAPABILITY OF EXPLOITATION IN INDUSTRY

As apparent from the foregoing description, a rapidly solidified thincast plate produced by a method in accordance with the present inventionand a sheet produced by processing the same thin cast plate are farsuperior to the conventional thin cast plate in mechanical propertiesand surface properties. Furthermore, the present invention provides anovel method of producing a material difficult to work, which has a highutility in industry.

We claim:
 1. A Ti--Al intermetallic compound sheet of a thickness in therange of 0.25 to 2.5 mm formed by processing a thin cast plate of aTi--Al intermetallic compound of 40 to 53 atomic percent of Ti, 0.1 to 3atomic percent of at least one material selected from the groupconsisting of Cr, Mn, V and Fe, and the balance Al and unavoidableimpurities, having in an as-cast solidified state, a columnar crystalstructure which consists of the <111> crystal orientation orientedpreferentially in the direction from the opposite surfaces of the castplate toward the central portion of the same, and is formed of a refinedcomposite structure of L1₀ structures and D0₁₉ structures.
 2. A Ti--Alintermetallic compound sheet according to claim 1, wherein the as-castsolidified thin cast plate has a mixed structure of a columnar crystalstructure extending from opposite surfaces thereof toward a centralportion thereof, and equiaxed crystals existed in a vicinity of thecentral portion thereof.
 3. A method of producing a Ti--Al intermetalliccompound sheet, comprising steps of: pouring a melt of the Ti--Alintermetallic compound of 40 to 53 atomic percent of Ti, 0.1 to 3 atomicpercent of at least one of material selected from the group consistingof Cr, Mn, V and Fe, and the balance of Al and unavoidable impuritiesinto the mold of a twin-drum continuous casting machine in an inert gasatmosphere and casting the thin cast plate of a thickness in the rangeof 0.25 to 2.5 mm by cooling the melt by the two drums and cooling thecast plate immediately after the cast plate has left the two drums to atemperature not higher than 200° C. at a cooling rate not higher than200° C./sec.
 4. A method of producing a Ti--Al intermetallic compoundsheet according to claim 3 further comprising a step of subjecting thecast plate cooled to a temperature not higher than 200° C. to hotisostatic pressing at an atmosphere of a temperature of 1000° C. orhigher and a pressure of 1000 atm or higher.
 5. A method of producing aTi--Al intermetallic compound sheet according to claim 3, wherein thecast plate is cooled by the two drums at a cooling rate in the range of10² ° C./sec to 10⁵ ° C./sec.
 6. A method of producing a Ti--Alintermetallic compound sheet according to claim 4 further comprising astep of hot working at a temperature in the range of 1200 to 1400° C. ata low strain rate of 5×10⁻⁴ /sec or below after carrying out the hotisostatic pressing to the cast plate.
 7. A method of producing a Ti--Alintermetallic compound sheet according to claim 3 or 4, the cast plateis held at a temperature in the range of 800° to 1000° C. for a time inthe range of 1 to 20 minutes immediately after the cast plate has leftthe two drums, and then the cast plate is cooled to a room temperatureat a cooling rate of 200° C./sec or below.